A storage system generally provides a logical volume, which has been created based on a RAID (Redundant Array of Independent Disks) group comprising multiple storage devices, to a higher-level apparatus (for example, a host computer). In recent years, a nonvolatile semiconductor storage device comprising multiple nonvolatile chips, has been employed either in addition to or instead of a HDD (Hard Disk Drive) as the storage device. For example, a flash memory, which comprises multiple flash memory chips (hereinafter, FM chips), is employed as the nonvolatile semiconductor memory device (for example, Patent Literature 1).